This invention concerns a device to move the rolls in a four-high rolling stand to roll sheet and/or wide plate.
The device is applied advantageously in the siderurgical field in operations to position and move the rolls in rolling mill stands to roll sheet and/or wide plate.
In rolling mill stands for sheet and/or wide plate, the state of the art covers the procedure of shifting the coupled working rolls, or one individual roll, in order to distribute the localised wear uniformly, in particular in correspondence with the edge of the rolled stock, and/or to vary the section geometry of the rolled product when shaped rolls are used.
The rolls are shifted and held in position by means of elements connected to a drawing system, having gripping means of a shape suitable to associate with seatings situated on the chocks of the rolls on which the gripping means act.
The gripping means can be engaged or disengaged from the relative seatings and therefore from the chocks, by means of control systems of various types, allowing the roll to be constrained to and released from the drawer every time it is necessary, as for example when the roll is taken out or inserted in the event of replacement or maintenance.
The state of the art also knows the method by which the thickness of the rolled product is controlled, during rolling, by moving the rolls with respect to the rolling plane in such a way as to vary the distance between the rolls.
This moving is carried out by means of mechanical or hydraulic devices with which the rolling stand is endowed.
The movements may take place even during the operating cycle, for example when the product is about to enter the rolling stand.
Each of the two working rolls or at least one of them can therefore be subjected to perpendicular shifting movements in respect to the rolling plane while the surfaces of the chocks are in close contact with the relative gripping means to execute the desired axial shifts.
These movements which are at right angles to the rolling plane cause a slip between the contact surfaces of the gripping means on the drawing device and the mating seatings on the chocks, which in turn determines friction which increases according to the intensity of the axial thrusts given by the working rolls.
The thrusts are greater when there are defects of parallelism between the working rolls and/or their generators, as they tend to generate a reciprocal tightening movement of the rolls.
The slip itself may be caused, as we have said, by the impact with the rolling rolls of the leading end of the rolled product as it arrives. The less precisely the thickness of the product is controlled during rolling, and the greater the reduction of the thickness programmed by the specific rolling stand, the greater the slip.
Solutions have been devised where, during the working cycle, the working rolls undergo a crossing movement (roll-crossing) or where both the working rolls and the back-up rolls together undergo a crossing movement (pair-cross) This gives greater flexibility in controlling the profile of the rolled product.
The crossing of the rolls however causes an increase in the axial thrusts and therefore an increase in the forces of friction between the contact surfaces of the gripping means and those of the mating seatings of the chocks.
The forces of friction oppose the adjustment movement of the rolling rolls and cause considerable imprecisions in the system to control the thickness of the rolled product, and consequently an increase in the margins of tolerance of the thicknesses, which can exceed the limits of what is accepted by the market.
The forces of friction also cause a considerable wear of the contact surfaces where they are generated, further increasing the friction and reducing the efficiency of the gripping elements and therefore of the whole system by which the rolls are moved.